Kolb, who is locked in a competition with rookie first-round pick EJ Manuel, avoided serious injury to his left knee when he slipped and stumbled awkwardly while switching fields between practice drills Saturday.

Coach Doug Marrone said the injury does not appear to be serious. He called it "a bump," and listed Kolb "day by day." The quarterback was held out of practice Sunday but could return Monday, Marrone said.

Marrone also provided an update on the status of Stevie Johnson, saying the team's No. 1 receiver will miss at least a week with a pulled left hamstring. Johnson was hurt in practice Friday.

Kolb wasn't available for comment Saturday, but showed frustration by angrily tossing his helmet to the ground before being examined by head trainer Bud Carpenter. After a few minutes, Kolb got up on his own and was escorted to the locker room.

Kolb was joining the quarterbacks and receivers in making their way through a tunnel to work out on an adjoining field. He slipped on a mat laid down over the concrete, and immediately began hobbling in pain. Kolb then put his arm around Carpenter's shoulder for support.

It was a freak accident that happened about 50 minutes into practice at Bills training camp in suburban Rochester.

When wet, the mats provide very little traction, especially for players wearing cleats. Shortly after Kolb slipped, several camp volunteers arrived with brooms to sweep away any remaining water from the mats.

Manuel took the extra precaution the next time he went through the tunnel by walking mostly on the concrete.

This is but the latest in a series of setbacks for Kolb, who is entering his seventh NFL season and seeking a fresh start in Buffalo after signing a two-year contract in April.

Selected by Philadelphia in the second round of the 2007 draft, Kolb was sidelined by a concussion in losing the Eagles' starting job to Michael Vick to open the 2009 season. Kolb then spent the past two seasons in Arizona, where he failed to secure the starting job because of an assortment of injuries.

Marrone said it's unfair to label Kolb injury-prone.

"The one thing I do know about Kevin is that he is a tough son of a gun," Marrone said. "He'll get back out there quickly if it's up to him."

In the meantime, Manuel will be getting more practice time with the starters. That's a significant step up after Kolb spent much of the first week of camp getting most of the work with the starters.

Marrone had been increasing Manuel's responsibilities over the past week, with the two quarterbacks nearly evenly splitting snaps by Friday. Marrone liked how Manuel responded after Kolb was hurt by not letting the injury affect him or the offense.

"It was a very smooth transition," Marrone said. "It can affect the team. And I didn't see it affect the two guys behind him, so that's positive."

It's too early to determine whether Kolb's injury will affect the quarterback rotation for the Bills preseason opener at Indianapolis on Aug. 11.

Manuel has shown improvement with each day of practice. On Saturday, he hit Marcus Easley on a deep route, and also showed off his speed in taking off on quarterback sweeps.

Manuel was selected 16th overall out of Florida State, and the only quarterback taken in the first round of the draft.

The Bills have only one other quarterback at camp, Jeff Tuel, an undrafted rookie free agent out of Washington State.

As for Stevie Johnson, the receiver was disappointed to be sidelined once again by injury. He missed most of the team's spring practices with a sore back. He's also had a history of groin problems that have cropped up during camp.

Johnson said he felt something pop just as he was pushing off his left leg to make a cut. He expects to be ready for the start of the season.

"The season's where the money's made, man," Johnson said. "I'll be out there to play."